The present invention relates to an objective lens employed in an optical disc drive for recording information to and/or reproducing information from a plurality of types of optical discs having different recording densities.
There are various types of optical discs, such as a CD and a DVD, having different recording densities and different thicknesses of cover layers. Recently, a new technical standard optical disc, such as an HD DVD or a BD (Blue-ray Disc), having a recording density higher than that of the DVD is becoming commercially practical. Such a new technical standard optical disc has a cover layer thickness smaller than or equal to that of the DVD. Considering a usability for a user of these type of optical discs (i.e., three types of optical discs), an optical disc drive (i.e., an optical system for the optical disc drive) capable of supporting all of the three types of optical discs (i.e., to have compatibility with the three types of optical discs) is desirable.
The term “optical disc drive” as used herein includes an optical disc drive designed specifically for recording information to an optical disc, an optical disc drive specifically designed for reproducing information from an optical disc, and an optical disc drive having both of functions of recording information to and reproducing information from an optical disc, The expression “an optical disc drive has compatibility with the three types of optical discs” as used herein means that a recording operation and/or a reproducing operation are guaranteed for all of the three types of optical discs, without the need for replacement of parts in the optical disc drive.
In order to support a plurality of types of optical discs of different technical standards, it is required to change a numerical aperture (NA) of light used for recordation or reproduction of information while compensating for spherical aberration which varies depending on a thickness of a cover layer of an optical disc being used so that a beam spot having a diameter suitable for a recording density of an optical disc being used can be achieved. A beam spot diameter decreases as a wavelength of a laser beam decreases. Therefore, an optical disc drive generally uses a plurality of types of laser beams having different wavelengths.
For example, a laser beam having a wavelength of approximately 790 nm is used for the CD, and a laser beam having a wavelength of approximately 660 nm which is shorter than that for the CD is used for the DVD. A laser beam having a shorter wavelength (e.g., approximately 408 nm which is so-called blue laser) than that for the DVD is used for the new technical standard optical disc because of its recording density higher than that of the DVD.
In order to converge a laser beam onto a recording surface of each of the plurality of types of optical discs, one or a plurality of optical elements forming an optical system of an optical disc drive is provided with an annular zone structure on one lens surface in the optical system. The annular zone structure formed on the entire region of the lens surface includes minute steps formed between adjacent annular zones so that each of the laser beams having different wavelengths can be converged onto a recording surface of each of the plurality of types of optical discs by the effect of the annular zone structure.
It is preferable that the optical element provided with the annular zone structure has a function of compensating for spherical aberration caused when a wavelength of a laser beam shifts from its design wavelength. The term design wavelength means a wavelength of a laser beam suitable for recordation or reproduction of information for each of the plurality of types of optical discs.
In Japanese Patent Provisional Publication No. 2004-247025, an optical pick-up supporting all of the CD, DVD and HD DVD is disclosed. In the optical pick-up disclosed in the publication, an objective lens is provided with an annular zone structure, for which third-order diffracted light is used for recordation or reproduction of information for an optical disc having the highest recording density and second-order diffracted light is used for recordation or reproduction of information for the DVD or CD. By employing such an objective lens, the optical pick-up is able to form a beam spot suitable for recordation or reproduction of information for each of the plurality of types of optical discs. By this configuration, the optical pick-up achieves the function of supporting the three types of optical discs having different recording densities.
However, the optical pick-up disclosed in the publication has a drawback that the optical pick-up can achieve only 40% usage efficiency of light for recordation or reproduction of information for the CD and undesired diffraction order light (e.g., first-order diffracted light in this case) having the light quantity corresponding to 40% usage efficiency of light is undesirably generated. By such a drawback, a waveform of a focus error signal may be deformed, thereby decreasing focusing performance. Further, in this case a suitable beam spot diameter can not be achieved.